If you go into Computer Management, Disk Management, does it show the USB device there? Is the Drive Letter that is listed there the same as one of your mapped drive letters? That could be an issue, just right click it in the Disk Management and change Drive letter and it will work again if this is the case.
It could also be a driver issue, what OS are you running and what kind of USB, make sure that you do not need drivers for that OS if you do install them and it should work.
Hope this helps.